1. Technical Field
The present invention relates to devices used for the measurement of aerosol particles and, in particular, the size distribution and mobility of such particles.
2. Background Art
Differential mobility analyzers (DMA) have been used to classify and analyze aerosol particles. The measurement of such particles has substantial commercial importance, including the analysis of atmospheric pollutants, the characterization of contaminants in laboratories and controlled production environments, the calibration of aerosol instruments, and the separation of particle sizes to produce monosized particles. Aerosol particles may be composed of various organic and inorganic materials and vary in size from one nanometer to approximately 100 microns.
The classification of aerosol particles by a DMA is based on the property that such particles carry electrical charge. An electrostatic field is applied by the DMA and positive and negative charges migrate or deflect under the field toward the electrode with an opposite electric charge. One common DMA employs coaxial cylinders as electrodes in a two-inlet, two outlet flow configuration (Hewitt, 1957). Although a commercialized version has been widely used for the generation of monodisperse particles and the size classification of polydisperse aerosols, the high cost of precision machining to produce the cylinders is a significant disadvantage.
One approach to reducing the cost of the DMA is the use of a circular configuration for the electrodes, such as the designs disclosed in the cumulative mobility analyzer of Hurd and Mullins (1962) and the mobility classifier of U.S. Pat. No. 5,117,190 (Pourprix). These devices have reduced the cost, size and weight of such analyzers, but continue to exhibit other drawbacks. For example, in the DMA disclosed by Pourprix, the analyzer extracts an aerosol sample in such a manner that the number of particles detected for specific test conditions (such as flow rate and applied voltage) will be produced by aerosol particles with a wide size distribution rather than a narrow one, which can result in an inaccurate size distribution. As shown in FIG. 8 of U.S. Pat. No. 5,117,190, the aerosol sample is extracted through an annular slot 76 and into a cylindrical case 82, before the sample is extracted through a tube 84 for analysis. The collection of particles in the case 82 results mixing or diffusion of particle sizes produced during one set of test conditions with those of a previous set of test conditions, which may be referred to as a "time hysteresis effect." This time hysteresis effect reduces the resolution of the DMA when the applied voltage is varied either continuously or incrementally by rapidly stepping through values.
In addition, the DMA disclosed in the Pourprix patent experiences difficulty in measuring ultrafine aerosol particles, for example less than 10 nm. This is primarily due to aerosol particle losses in the entrance slots, which produce a radial flow of aerosol that flows in a centripetal manner toward the center of the circular electrodes.